1. Field of the Invention
The present invention relates to the fabrication of printed circuit boards and, more particularly, concerns an apparatus for, and a method of, assembling copper-caul-copper elements used in the fabrication of printed circuit boards.
2. Description of the Related Art
Printed circuit boards are used in most electronic applications. In one of its simplest forms, the printed circuit board has two sides with conductive traces that are separated by an insulating layer. Electronic devices, such as transistors, resistors, and the like, are positioned on the first side of the circuit board so that the traces interconnect the different devices. However, it may be difficult to interconnect two devices on one side of a board having multiple devices and multiple traces while maintaining isolation between different sets of devices. Consequently, traces are also formed on the second side of the printed circuit board in positions where holes can be drilled through the insulating layer to the traces on the second side to thereby interconnect devices on the first side of the printed circuit board. In more complex printed circuit boards, the insulating layer is comprised of multiple layers of pre-preg material where conductive traces are also formed in the prepreg material. The multiple layers of pre-preg boards are separated from each other by insulating layers, and holes of varying depths are drilled into the multiple layers to interconnect devices using the traces formed on the pre-preg boards.
A common method of forming printed circuit boards, including multiple layer printed circuit boards, begins with positioning a thin layer of copper on a plate known as a caul. The caul is generally a steel plate that has been finely milled so that it is extremely flat. This allows the copper layer to be positioned on the caul so that the copper layer lies substantially flat on the caul. It is understood that the layer of copper can be very thin, e.g., on the order of one or two mils thick, which requires that the caul be milled to a very fine tolerance so that the copper layer will rest uniformly on the caul.
Subsequent to the layer of copper being positioned on the caul, one or more layers of insulating material or prepreg material are then positioned on the upper surface of the layer of copper. Preferably, the layer of copper has been treated so that one side is covered with an epoxy that will subsequently adhere to the layer of insulating material. Once the layer of insulating material has been positioned on the layer of copper, another layer of copper is then positioned over the layer of insulating material, with another caul material positioned on top of the second layer of copper.
In general, multiple layers of cauls, copper, and insulators are assembled in this fashion into what is generally referred to as a book. The book of copper layers, cauls, and insulating materials is then inserted into a composite board press, such as the composite board press described in U.S. Pat. No. 4,857,135. In this type of press, the book is positioned between a pair of heated platens wherein the book is simultaneously heated and compressed by the platens. Further, the composite board press is typically under vacuum, and the combined effects of the heating, compression and vacuum causes resin in the insulating material to flow uniformly and thereby uniformly adhere the copper layers to the insulating material.
Subsequently, the book is removed from the oven and is separated into printed circuit boards which are comprised of two uniform layers of copper separated by the insulating layer. The two uniform layers of copper can then be lithographically etched so as to define the traces used to interconnect devices that are then positioned on the board.
The assembly of the books is thus a critical component in the fabrication of printed circuit boards. In order to reduce the costs of fabricating printed circuit boards, it is desirable to automate the book assembly process. Generally, previous approaches to automate the book assembly process have resulted in the copper being cut and then positioned on the caul at a first station, and then the caul and copper being moved to a second station wherein the insulating layer or multiple layers of pre-preg is then positioned on top of the copper. Subsequently, the element would then be moved either back to the first station or to yet a third station, wherein another layer of copper and a caul is then positioned on top of the insulating layer(s). However, this particular approach is inefficient and expensive in that it requires the book to be moved back and forth between multiple stations and may even require multiple stations performing nearly identical functions.
Further, the insulating layer is generally comprised of glass fibers that are bound within an epoxy mesh. The insulating layer can introduce a large amount of contaminants, e.g., dust and the like, into the environment where the layers of insulating layers are being positioned on the copper. These contaminants can come to rest on a caul prior to the positioning of a layer of copper on the caul, which results in non-uniformity of the layer of copper, i.e., the layer of copper becomes nonplanar. This can further result in the layer of copper not adhering to the epoxy at particularized locations on the printed circuit board. This problem is accentuated where very thin layers of copper and extremely small traces are being formed from the very thin layers of copper, as the very thin traces will be more likely to break at places where they are not adequately adhered to the insulating layer.
Hence, there is a need for a process and apparatus that assembles books for printed circuit boards wherein copper plates can be positioned on cauls in an efficient manner. Further, there is a need for a method and apparatus that provides efficient assembly of cauls and copper layers while reducing the risk of contamination as a result of subsequent positioning of insulating layers on the layers of copper.